ABSTRACT: Diel and spatial variability in the concentration of virus-like particles (VLP) was determined in a transect from the coast to offshore in the western Mediterranean (7 stations and 6 depths per station). VLP abundance in coastal waters was on
average 1.6 x 107 VLP ml-1, in offshore stations this value was 1.2 x 107 VLP ml-1. At the coastal station there was no varaibility in VLP abundance, bacterial abundance or chlorophyll a concentration
through the water column, while offshore VLP decreased with depth. No clear diel pattern was found in 3 representative stations where diel cycles were studied. Viral impact on the bacterial assemblage was estimated at 2 depths (surface and deep
chlorophyll a maximum) of the latter 3 stations using 2 approaches: the percentage of visibly infected bacteria and viral decay rates in cyanide amended cultures. Visibly infected bacteria could not be detected in any sample after counting 300
cells per sample, implying that the percentage of visibly infected bacteria was lower than 0.3%. Using the maximal conversion factor to convert this value to bacterial mortality we found that viruses could be responsible at most for 21.5% of the whole
bacterial mortality. We calculated a hypothetical viral decay rate if the maximal possible impact of viruses (21.5%) were true. This value corresponded to 0.02 h-1. However, using the cyanide method to measure viral decay rate, no significant
decrease could be detected in a total of 8 viral decay experiments. In conclusion, the percent of total bacterial mortality due to viral lysis in the oligotrophic northwestern Mediterranean was low, certainly much lower than 20%.